Various proposals have been made so far as to polymerization catalysts for conjugated dienes, and they play a highly important role in industrial fields. In particular, various polymerization catalysts which give a high cis-1,4-linkage content have been studied and developed to obtain conjugated diene polymers with enhanced performance in thermal and mechanical properties. For example, complex catalyst systems containing a compound of a transition metal such as nickel, cobalt and titanium as a main component are known, and some of them have already been widely used in industrial applications as polymerization catalysts for butadiene, isoprene etc. (see, End. Ing. Chem., 48, 784, 1956; Japanese Patent Publication No. 37-8198).
In order to attain a higher cis-1,4-linkage content and superior polymerization activity, complex catalyst systems which consist of a rare earth metal compound and an organometallic compound belonging to Group I to Group III have been studied and developed, and highly stereospecific polymerization has come to be actively studied (Makromol. Chem. Suppl, 4, 61, 1981; J. Polym. Sci., Polym. Chem. Ed., 18, 3345, 1980; German Patent Application No. 2,848,964; Sci. Sinica., 2/3, 734, 1980; Rubber Chem. Technol., 58, 117, 1985 etc.). Among these catalyst systems, complex catalysts containing a neodymium compound and an organoaluminum compound as main components were revealed to give a high cis-1,4-linkage content and have superior polymerization activity. The catalysts have already been used in industrial applications as polymerization catalysts for butadiene etc. (see Macromolecules, 15, 230, 1982; Makromol. Chem., 94, 119, 1981).
With the recent progress of industrial technologies, requirements for polymeric materials as commercial products have become increasingly higher, and development of polymeric materials which have still higher thermal properties (thermal stability etc.) and mechanical properties (tensile modulus, bending modulus etc.) has come to be strongly desired. As one of promising means for achieving the object, there have been attempts to produce a polymer of a high cis-1,4-configuration content in microstructure and a narrow molecular weight distribution by using a catalyst having a high polymerization activity for conjugated dienes. However, no method has so far been found for producing polymers having such characteristics.
Further, various proposals have hitherto been made also as to catalysts for copolymerization of a conjugated diene and an aromatic vinyl compound, and they play an extremely important industrial role. In particular, various copolymerization catalysts which give a high cis-1,4-linkage content have been studied and developed to obtain copolymers of a conjugated diene and an aromatic vinyl compound with enhanced performance in thermal and mechanical properties.
For example, there are known complex catalyst systems containing a compound of transition metal such as nickel, cobalt and titanium (see Kogyo Kagaku Zasshi (Journal of Industrial Chemistry), 72, 2081, 1969; Plast. Kautsch., 40, 356, 1993; Makromol. Chem. Phys., 195, 2623, 199 etc.), complex catalyst systems containing a compound of rare earth metal such as neodymium and gadolinium (Macromol. Rapid Commun. 16, 563, 1992; J. Polym. Sci., ParA; Polym. Chem., 32, 1195, 1994; Polymer, 37, 349, 1996) and the like. Although these catalyst systems exhibit a relatively high cis-1,4-controllability, polymers with a high molecular weight and narrow molecular weight distribution, and copolymers with randomized monomer sequence cannot be obtained by means of these catalysts.